Head to the jail area within Najena's Hollow Tower and take a rock sample. The sample is in the small tunnel that connects the two jail cells, covered over by a clickable poster you can burn to open the tunnel. This is also where you will find the key to unlock the valves to gain access to the lower level of the tower. edit : loc for sample was -47, -195, -142 (in one prisonner cell, not in the tunnel between the two cells)

Deliver the sample to Isxot within the Thexian Dragoon camp in Lavastorm Mountains